Corals generally get their glow from fluorescent pigments that act as sunblock. The sun's intense rays, which can sunburn swimmers and divers that flock to these reefs, cause similar damage to coral and zooxanthellae, the symbiotic algae that lives inside coral.

Although bright sunlight at shallow depths can make the pigments hard to see with the naked eye, they can be visible if the coral makes lots of them, says Wiedenmann, who led a new study on the corals published June 24 in PLOS ONE.

Though these pigments are well studied, scientists hadn't looked much at fluorescence in deeper dwelling corals, since they're not as exposed to sunlight.

The fluorescence of this Lobophyllia coral can change from green to red when exposed to blue or ultraviolet light.

Photograph by Professor J Wiedenmann

The researchers found that some of the corals at these depths glowed an intense green or orange. After photographing them in their natural environment, Wiedenmann packed samples of 16 different species of coral into plastic bags and brought them back to his lab in England for further study.

When Wiedenmann illuminated the corals with blue or ultraviolet light—mimicking what's found in the ocean depths—he found that they could also glow red or green. Interestingly, Wiedenmann also discovered that the corals could produce these pigments in the absence of any light at all. (Watch: Go beneath the waves and see coral reefs in living color.)

This, combined with the general lack of sunlight at these depths, means that these pigments weren't acting as a sunscreen.

Instead, the researchers believe that the pigments help make more light for their symbiotic algae, which need it for photosynthesis. Happy algae translates to more oxygen and other benefits for the coral.

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Red Sea corals glow in a rainbow of hues, a process that likely boosts the algae living inside them.

Photograph by Professor J Wiedenmann

"To me, the most interesting part of the study is the range of colors you can find in very closely related species," Dimitri Deheyn, a marine biologist at the Scripps Institution of Oceanography who wasn't involved in the research.

He expected that similar corals would have similar colors, rather than the array that Wiedenmann found.

The corals' rainbow of hues is more than just a feast for our eyes—they may someday play a role in improving human health.

Wiedenmann says that these pigments could help scientists do everything from tagging certain types of cells to look at them under the microscope to helping physicians better see cancer cells in the body.